(1) Field of the Invention
The invention relates to the general technical field of aviation, and more precisely to the construction of aircraft structural elements, in particular for rotary wing aircraft such as rotorcraft. Such structural elements, that need to be distinguished from airfoil surfaces, often give rise to aerodynamic drag. When designing aircraft, and in particular rotorcraft, specifically helicopters, it is always desirable to reduce as much as possible the aerodynamic drag generated by the various structural elements making up the aircraft.
The present invention relates more particularly to a ducted tail rotor for a helicopter, known as a Fenestron®, or more generally a structural element that has a certain amount of thickness, and that is of a shape that generates aerodynamic drag.
(2) Description of Related Art
By way of example, certain ducted tail rotors of the Fenestron® type have a rear portion that is terminated by a clearly-defined base, e.g. by a part that extends substantially orthogonally relative to the forward direction and that closes the rear portion of the Fenestron fairing. Downstream from its clearly-defined base the fairing of such a tail rotor thus presents a width that suddenly becomes zero. Such a shape gives rise to a considerable amount of wake and is responsible for a non-negligible portion of the aerodynamic drag. Such a shape therefore penalizes the aerodynamic performance of the aircraft in forward flight.
The flow of air downstream from the clearly-defined base becomes massively separated from the structure and it is this aerodynamic separation, i.e. a relatively extensive zone in which turbulence and vortices are concentrated, that gives rise to a large amount of energy dissipation and also to vibration that is induced by aerodynamic forces.
In order to reduce those drawbacks, proposals have been made to replace the clearly-defined base with a portion of fairing that terminates in a profiled shape. Such a change of shape is found to be effective in terms of drag only if the relative thickness of the fairing is less than 15%.
The relative thickness is the value of the ratio between the maximum thickness of the fairing in the direction extending transversely to the flow divided by its length (the dimension that it occupies in the longitudinal direction of the flow).
Such a relative thickness requires a large amount of length (chord of the fairing), given the fairing needs sufficient width to enable it to house the anti-torque rotor. The use of such a profiled portion therefore increases the weight of the aircraft. This increase in weight is also located very far to the rear of the center of gravity of the aircraft, thereby giving rise to problems of centering the aircraft. Furthermore, such a profiled portion gives rise to problems in terms of flight quality, in particular heading instability of the aircraft. Such heading instability may indeed appear with a reduced drag in flight, but where the location of the separation of the airflow from the structural element is at a position that is unstable. Since the flapping of the wake is not localized in the same place, it gives rise to natural self-sustaining oscillations in the forces generated by the fairing, in particular in terms of yaw. The pilot can then sense yaw oscillations about the heading (route) as set by the pilot that come from the tail of the helicopter.
Such a profiled portion may also generate asymmetry in terms of lateral flight of the aircraft. The greater the lateral surface area of the fairing, the more it opposes movement during lateral flight. For example, lengthening the tail of a Fenestron causes it to offer a larger surface area to wind in lateral flight.
A rotorcraft rotor blade is also known, e.g. from document EP 0 724 691, that has aerodynamic portions in relief that improve the aerodynamic characteristics of the blade, in order to enhance the flow of the surrounding fluid. Those portions in relief are formed in particular by waves arranged in the general plane of the blade, both as projections and as depressions. Those waves give rise to variations in the thickness of the blade that are distributed along its span, both on its suction side and on its pressure side. Those portions in relief are also formed by varying the curvature of the blade in its depth direction, said portions in relief being arranged as sawteeth distributed along the span of said blade, at least at its leading edge, and possibly also at its trailing edge. That document therefore relates very specifically to airfoil surfaces of a rotorcraft.
Also known is the document “Near-wake flow dynamics from trailing edge spanwise perturbation” 4thConference on fluid control, Jun. 23-26, 2008, Seattle, Wash. That publication relates to a study of three-dimensional fluid flow at the rear of a thin plate that is subjected to periodic sinusoidal geometrical perturbations in the span direction. Such a plate therefore has a truncated trailing edge of sinusoidal shape in the span direction, thereby generating aerodynamic forces. The results of that study show that in comparison with a structure of the thin plate type having a truncated and flat trailing edge, thereby generating von-Karman-Bernard turbulence, sinusoidal perturbations significantly modify the structure of the wake and have a strong and favorable influence on the three-dimensional von-Karman-Bernard vortices.
By altering the geometrical shape of the base of the fairing, natural instabilities that are present in the wake are encouraged to grow. The vortices that are created in the portion of the wake that is very close to the base are therefore naturally caused to dissipate more quickly because of the geometrical modifications that give rise to aerodynamic perturbations.
The results of that document do not relate to structural elements of a rotorcraft that present a certain amount of width. In addition, the problems of drag induced by such perturbations are not addressed in that document.
In addition to the documents EP 0 724 691 and “Near-wake flow dynamics from trailing edge spanwise perturbation”, other documents should also to be taken into consideration.
Document GB 577 524 describes a rotary wing aircraft having an anti-torque tail rotor. Suction openings for the anti-torque rotor are provided on opposite side walls of a tail boom of the aircraft. The blades of a main rotor drive a stream downwards, which stream is forced into the fuselage through an air inlet under the main rotor, and is then channeled to the suction openings in the opposite side walls of the tail beam.
Document EP 1 527 992 describes an airfoil surface in the form of a wing with a flap arranged to create a vortex. Concave grooves are provided in the longitudinal direction of the aircraft.
Document JP 2000/255496 describes horizontal grooves in an anti-torque rotor fairing, the grooves all being parallel in a plane that is longitudinal and horizontal, and the grooves all being transversely continuous from one edge to the other of the fairing.
Other documents such as U.S. Pat. No. 6,345,791 and US 2008/0217484 describe aerodynamic airfoil surfaces having respectively surface wrinkles or angulations of the rear free edge.